Ultra-wideband (UWB) pulse technology has been at work for many years, but has traditionally been used in classified applications. However, with the increasing number of wireless applications, UWB is seen as a technology that can provide additional bandwidth utilization without contributing to spectral crowding.
UWB communication employs the technique of transmitting high frequency, narrow-duration impulses, referred to as monocycles, through the radio channel. This provides a very large signal bandwidth from which the name arises. UWB radio does not require base band modulation. This characteristic makes this mechanism very desirable because, unlike other radio technologies, it is carrier-less and, thus, provides the potential for reduced complexity and reduced cost. Although UWB promises to provide a viable, cost-effective, high-bandwidth, short-range radio communication channel solution there are considerable obstacles to overcome.
The example in FIG. 1 illustrates a multi-user transmission 10 where there are three users 20, 22, 24 each with a unique orthogonal time hopping code where each bit is represented by three pulses (i.e. 20a, 20b and 20c). A radio communicating with user 20 transmits three pulses, 20a, 20b and 20c, having a bit duration 12 and frame duration 14, for each bit sent in the time sequence designated to user 20. The receiver used by user 20 compares the received data against its designated time sequence until it finds a match then the receiver synchronizes to the received signal.
Once synchronized to the received channel, the receiver must then decode the modulated data. This design supports On-Off Keying (OOK) and Pulse Position Modulation (PPM). OOK is a type of modulation in which data pulses are switched on and off to modulate between 1 and 0 as shown in FIG. 2. PPM is a type of modulation in which the position of the monocycle is time-shifted to indicate a 1 or 0, as shown in FIG. 3.
As seen in the example above, impulse radio relies on a high precision timing sequence. The transmitter must broadcast pulses at precise time intervals constituting a specific time hopping sequence that repeats for each bit time. For a receiver to detect a broadcast, the receiver must generate its own local timing sequence that matches that of the transmitter. The receiver uses its local sequence to determine precisely the times that pulses are expected. The receiver then checks for pulses at each of these sequence times and then sums the total number of pulses detected over the length of the code sequence. This sum gives an indication of whether or not a bit is present.